Engines may use various forms of fuel delivery to provide a desired amount of fuel for combustion in each cylinder. One type of fuel delivery uses a port injector to deliver fuel to respective cylinders. Still another type of fuel delivery uses a direct injector for each cylinder.
Engines have been described utilizing multiple injector locations with different fuel types. One example is described in the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. Specifically, the Heywood et al. papers describes directly injecting ethanol to improve charge cooling effects, while relying on port injected gasoline for providing the majority of combusted fuel over a drive cycle.
The inventors herein have recognized that under some conditions, such as where one of the injectors is not continuously injecting a fuel or other substance, subsequent operation of the injector may become degraded where injector fouling occurs. As one example, a direct injector may become fouled where soot or other products of combustion form on or around the injector nozzle, thereby reducing injector performance.
The inventors herein have also recognized that the temperature of the injector may increase during periods where the injector is not performing an injection since the injection of a substance can cool the injector and/or stagnation of the substance downstream of the injector may remove less heat from injector. If the period between injection events becomes too great, the injector can exceed a temperature threshold at which injector degradation and/or failure occurs. Further, at high injector temperatures, the substance passing through the injector may be damaged or may foul the injector.
Thus, in one approach, a method of operating an engine of a vehicle is provided wherein the engine has a variable engine output. The method comprises during a first engine output, delivering a first fuel to at least a cylinder of the engine via a first injector and directly injecting a second fuel directly into said cylinder via a second injector; and during a second engine output lower than said first engine output, delivering said first fuel to said at least a cylinder of the engine via said first injector and directly injecting at least a purging substance via said second injector based on an idle period since a previous injection by said second injector. In this way, fouling or thermal degradation of the injector may be reduced by periodically purging the injector with a purging substance.
Additionally, the inventors have recognized that purging of an injector may cause variation in the oxygen content and/or caloric content of the fuel mixture delivered to the engine. Thus, in situations where purging is utilized to reduce injector fouling or thermal degradation, undesirable variations or departures from a requested air/fuel ratio of the cylinder charge and/or torque production by the engine may occur.
Thus, the method may further vary an amount of the first fuel that is injected via the first injector responsive to an amount of the purging substance that is injected via the second injector. In this way, the requested air/fuel ratio and engine torque output may be achieved while reducing injector fouling and thermal degradation.